Paper is made of individual fibers which are deposited in a continuous sheet. The sheet is typically formed from a papermaking stock comprised of less than 1 percent wood fibers dispersed in over 99 percent water. The fibers and water are deposited onto a wire screen or screens in the former section of the paper machine to form a continuous web of paper. The papermaking stock is first fed to a headbox which distributes the stock across the width of the forming screen or screens on which the paper web is being formed. The headbox discharges the stock through a long narrow converging nozzle or slice which injects the stock onto the rapidly moving wire screen or between two screens. The fibers are retained on the wire surface while the majority of the water is drawn through the screen or screens. The former may be a single wire horizontal former (fourdrinier) or a two wire (twin wire) former The paper web thus formed is pressed, dried and wound into reels. The reels of paper formed on the papermaking machine are then further processed to produce smaller rolls or sets of paper for printing. Individual sheets are also made which may be used in sheet-fed printing presses, in copy machines, and in laser printers.
Because paper is made of individual paper fibers which are joined together during the pressing and drying process, the orientation of the fibers within the paper controls the physical properties of the paper. In particular, fiber orientation influences the strength and dimensional stability of the paper. It has been found that paper which has insufficiently uniform fiber orientation, when exposed to heat or moisture, will form more wrinkles or become more wavy than normal. Exposing paper to heat or moisture causes the paper to shrink or expand. It is the non-uniformity of the dimensional changes which causes the paper to wrinkle or ruck. Non-uniformities in the paper are in turn caused by fiber alignment streaks and other defects caused by non-uniformity of the flow of stock onto the wire or wires.
Printing presses, converting equipment and papermaking machines are increasing in speed. This means they are more sensitive to small instabilities in the paper web such as those caused by non-uniform dimensional changes in the paper. The instabilities can lead to web breaks or print quality problems. The printing industry in newspapers, magazines and books continues to use more and more color which results in more water or other liquids coming in contact with the paper web where they can release dried-in stresses which bring out the dimensional instability of the paper and cause it to wrinkle. At the same time, increased moisture decreases the paper strength making it more subject to breaking.
Further, the consuming public has come to expect not only more color printing but printing of higher quality. Slight cockling or warping of the paper can lead to unprinted areas. Where glossy paper is utilized, waviness or cockle results in non-uniform reflection which is distracting to the consumer.
The fact that a sheet or web of paper can become wavy upon exposure to moisture or heat has thus become of greater concern. Most processes which form an image upon paper employ heat or moisture. When paper in sheet form is processed through a photocopier, laser printer, or printing press, warping of the sheet may cause it to jam the machine and cause a significant loss of productive time. When paper in the form of a continuous web becomes wrinkled, it is liable to break. Breakage of a web within a printing press, in a winder, or on a coater, can cause significant down time as well as the loss of significant quantities of paper.
The problem of dimensional changes in finished paper is aggravated by the trend to use lower base weight paper to hold down paper costs. Lighter grade papers are more subject to press breakage or jamming. A lighter grade of paper also means that for a given amount of moisture transferred by printing, particularly of colored images, a greater percentage of moisture is introduced into the paper. The increased productivity of modern equipment means that even limited down time to clear a jam or rethread a broken web can have significant economic consequences in terms of lost production. Further, paper must lie flat for easier handling, loading and compact transportation.
The papermaking machine headbox and the slice contribute significantly to the uniformity with which the fibers are laid down to form a paper web. Improvements in headbox design are essential to meet the growing expectations of paper consumers for flatter, more dimensionally stable paper.
Various means for controlling flow and scale of the turbulence produced in a headbox between the stock input header and the slice gap or opening are known. One known type of headbox employs a bank of parallel tubes which employ small scale turbulence generators and pressure drop features to assure a more uniform flow of stock into the nozzle and from the slice opening onto the forming wire.
A headbox is shown in U.S. Pat. No. 4,898,643 to Weisshuhn, et al. which employs two series connected tube banks which are separated by an intermediate space which is connected to a control means. The second set of diffuser tubes connects the intermediate space with the slice by means of a diffuser tube system which appears to converge towards the nozzle. Weisshuhn, et al. do not disclose continuous banks of tubes extending between the headbox and the slice which converge.
What is needed is a headbox which deposits a more uniform mat of fibers onto a forming wire.